THE MILKY WAY GALAXY: INDIAN CONSTELLATIONS

 Indian constellations:

 In Hindu/Vedic astronomy Rashi means constellation. The twelve rashis along the ecliptic correspond directly to the twelve western star signs. These are however divided into 27 Nakshatras, or lunar houses.

THE MILKY WAY GALAXY: CHINESE CONSTELLATIONS

Chinese constellations:

 Chinese constellations are different from the Western constellations, due to the independent development of ancient Chinese astronomy. Ancient Chinese sky watchers divided their night sky in a different way, but there are also similarities. The Chinese counterpart of the 12 western zodiac constellations are the 28 "Xiu" or "mansions" (a literal translation).

THE MILKY WAY GALAXY: DARK CLOUD CONSTELLATIONS

Dark Cloud Constellations:

 The "Emu in the sky", a 'constellation' defined by dark clouds rather than the stars. A western interpretation would recognise the Crux or Southern Cross, on the left Scorpius. The head of the emu is the Coalsack. Members of the Inca civilization identified various dark areas in the Milky Way as animals, and associated their appearance with the seasonal rains. These areas are commonly referred to by modern researchers as dark cloud constellations or dark nebulae. Australian Aboriginal astronomy also used dark nebulae in some constellations, the most famous being the "emu in the sky" whose head is formed by the coalsack.

THE MILKY WAY GALAXY: AGE

Age:

 It is extremely difficult to define the age at which the Milky Way formed, but the age of the oldest star in the Galaxy is yet to be discovered, HE 1523-0901, is estimated to be about 13.2 billion years, nearly as old as the Universe itself. This estimate is based on research by a team of astronomers in 2004 using the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure, for the first time, the beryllium content of two stars in globular cluster NGC 6397. From this research, the elapsed time between the rise of the first generation of stars in the entire Galaxy and the first generation of stars in the cluster was deduced to be 200 million to 300 million years. By including the estimated age of the stars in the globular cluster (13.4 ± 0.8 billion years), they estimated the age of the oldest stars in the Milky Way at 13.6 ± 0.8 billion years. Based upon this emerging science, the Galactic thin disk is estimated to have been formed between 6.5 and 10.1 billion years ago.

THE MILKY WAY GALAXY: SIZE

 Size:

 The stellar disk of the Milky Way galaxy is approximately 100,000 light-years (9.5×1017 km) in diameter, and is believed to be, on average, about 1,000 light years (9.5×1015 km) thick. It is estimated to contain at least 200 billion stars and possibly up to 400 billion stars, the exact figure depending on the number of very low-mass stars, which is highly uncertain. Extending beyond the stellar disk is a much thicker disk of gas. Recent observations indicate that the gaseous disk of the Milky Way has a thickness of around 12,000 light years (1.1×1017 km)—twice the previously accepted value. As a guide to the relative physical scale of the Milky Way, if it were reduced to 130 km in diameter, the Solar System would be a mere 2 mm in width. The Galactic Halo extends outward, but is limited in size by the orbits of the two Milky Way satellites, the Large and the Small Magellanic Clouds.

THE MILKY WAY GALAXY: STRUCTURE

Structure:

 The disk of the Milky Way exhibits a spiral structure, which shows up in the distribution of the objects populating the disk component. These objects include, in particular, young stars, diffuse star-forming nebulae and open star clusters. The origin of the spiral structure is thought to be in density waves which are triggered by gravitational disturbances, in particular during encounters with neighbor galaxies. The density waves first concern interstellar matter, which is compressed, forms diffuse nebulae which become star forming regions, and later form clusters and associations of young stars, the most luminous and conspicuous of which are massive, hot, blue and short-lived. The stars which are even older have dissipated into a more diffuse background; this includes intermediate population I stars, older star clusters and the younger representatives of the planetary nebulae.